18 



REPORT — 1 884. 



* subsidence and gradual tliinning of the film,' because tliey maintain, 

 their horizontal position when the glass is turned i-ound its axis. The 

 experiment is both easy and interesting ; but the conclusion drawn from 

 it cannot be accepted. The fact is that the various parts of the film 

 cannot quickly alter their thickness, and hence when the glass is rotated 

 they re-arrange themselves in order of superficial density, the thinner 

 parts floating np over, or through, the thicker parts. Only thus can the 

 tendency be satisfied for the centre of gravity to assume the lowest 

 possible position. 



When the thickness of a film falls below a small fraction of the length 

 ot a wave of light, the colour disappears and is replaced by an intense 

 blackness. Professors Reinold and Riicker have recently made the re- 

 markable observation that the whole of the black region, soon after its 

 formation, is of uniform thickness, the passage from the black to the 

 coloured portions being exceedingly abrupt. By two independent 

 methods they have determined the thickness of the black film to lie 

 between seven and fourteen millionths of a millimetre ; so that the 

 thinnest films correspond to about one-seventieth of a wave-length of 

 light. The importance of these results in regard to molecular theory is 

 too obvious to be insisted upon. 



The beautiful inventions of the telephone and the phonograph, although 

 in the main dependent upon principles long since established, have imparted 

 a new interest to the study of Acoustics. The former, apart from its uses 

 in every-daj' life, has become in the hands of its inventor, Graham Bell, and 

 of Hughes, an instrument of first-class scientific importance. The theory 

 of its action is still in some respects obscure, as is shown by the compara- 

 tive failure of the many attempts to improve it. In connection with some 

 explanations that have been offered, Ave do well to remember that molecular 

 changes in solid masses are inaudible in themselves, and can only be 

 manifested to our ears by the generation of a to and fro motion of the 

 external surface extending over a sensible area. If the surface of a solid 

 remains undisturbed, our ears can tell us nothing of what goes on in the 

 interior. 



In theoretical acoustics progress has been steadily maintained, and 

 many phenomena, which were obscure twenty or thirty years ago, have 

 since received adequate explanation. If some important practical ques- 

 tions remain unsolved, one reason is that they have not j'et been definitely 

 stated. Almost everything in connection with the ordinary use of our 

 senses presents peculiar difficulties to scientific investigation. Some 

 kinds of information with regard to their surroundings are of such para- 

 mount importance to successive generations of living beiu^^, that they 

 have learned to interpret indications which, from a physical point of 

 view, are ot" the slenderest character. Every day we are in the habit of 

 recognising, without much difl[icnlty, the quarter from which a sound 



